1. Field of the Invention
The present invention relates to infrared detection systems and imaging systems. More particularly, the present invention relates to infrared detection and imaging systems employing either noncooled or cryogenically cooled focal plane arrays.
2. Description of the Prior Art and Related Information
Infrared imaging systems have a variety of applications ranging from military applications to commercial applications such as home and business surveillance systems and manufacturing inspection systems. While military applications have been driven by a desire for high performance with less emphasis on cost, the rapidly developing commercial applications require good performance at reasonable cost along with good reliability. Therefore, maximizing these various aspects of an infrared imaging system is a key to creating a commercially viable product. Nonetheless, significant problems arise in achieving this goal since tradeoffs are required between performance, cost and reliability. Such considerations typically involve the two key components of an infrared imaging system: the infrared detector itself and the readout circuitry for converting the detector signals to image data which may be effectively processed into a high quality image.
One approach to infrared detector design employs a so-called focal plane array composed of a large number of detector elements arranged in a two dimensional array. An infrared focusing lens is employed to focus the incident infrared energy into an image on the array of detector elements. Thus, the detected infrared energy at each detector element in the focal plane array corresponds to a picture element (pixel) of the infrared scene to be imaged. The focal plane array will typically have a relatively large number of individual elements, or pixels, the number being related to the image quality.
To minimize manufacturing costs and to maximize uniformity of the detector elements in the focal plane array, it is highly desirable to form the array of detector elements on a single monolithic integrated circuit. Each element in turn must be biased to give a suitable detection signal and each detection signal must be separately read out. Since the integrated circuit chip in which the detector elements are formed is typically quite small, the readout wiring can become a problem due to space availability on the chip and pinout constraints. To accommodate these considerations, some type of intelligent readout circuitry is typically provided on the focal plane array itself. The form in which the readout circuitry is integrated with the focal plane array detector will depend upon the specific detector array structure.
For cryogenically cooled focal plane array detectors employing photoconductor or photovoltaic detection, the readout circuitry is typically formed in a separate integrated circuit which is “bump bonded” to the detector integrated circuit to form a single hybrid detector/readout structure. An alternate approach incorporates the detector and the readout circuitry in a monolithic integrated circuit. Such monolithic structures may be used, for example, for microbolometer infrared detector arrays. In microbolometer infrared detectors, the incident infrared energy is detected by measuring a change in resistance in the microbolometer caused by a temperature increase or decrease due to the incident infrared energy. In a monolithic microbolometer array, an array of separate microbolometers is formed on top of a readout integrated circuit which acts as a support substrate.
A consideration which is extremely important for maximizing the quality of the output image from a focal plane array infrared detector is compensating for nonuniformity in the individual detector elements in the array. Whereas ideally a uniform temperature scene directed to the focal plane array would produce a completely uniform output at each pixel, in practice, the output of the individual detector elements may vary by a significant percentage of the average output level. When an actual scene is detected, such detector element nonuniformity can significantly degrade the image quality or even completely mask the actual image. This degrading of the image due to detector element nonuniformity is sometimes called spatial or fixed pattern noise.
Although any focal plane array infrared detector will have variation in the average or DC response of the individual detector elements, and hence suffer from fixed pattern noise, this problem is particularly severe in the case of microbolometer focal plane arrays and specifically focal plane arrays employing microbridge-type microbolometer detector elements. Such nonuniformity problems are due in part to the fact that the above noted microbolometer detectors are adapted to operate at noncryogenic (i.e., noncooled) temperatures as opposed to the cryogenically cooled photoconductor and photovoltaic type detectors. Also, the fabrication technology employed in microbridge microbolometer detectors inherently introduces higher degrees of nonuniformity than is present in cryogenically cooled detection systems.
The problem of nonuniformity in the detector elements is directly related to issues of manufacturing throughput and cost. That is, if very high uniformity is required for the detector arrays to ensure good image quality, the number of focal plane arrays which must be rejected will increase on average. This in turn reduces throughput, increasing per unit costs. Therefore, it is generally preferable for nonuniformity to be tolerated by virtue of compensation in the detector electronics rather than controlling nonuniformity during detector array fabrication.
In prior art approaches to compensating for the nonuniformity in infrared focal plane arrays, and in particular in microbolometer infrared focal plane arrays, the nonuniformity has been corrected utilizing digital signal processing electronics configured on a separate integrated circuit, normally separate printed circuit board, from the focal plane array itself. Since the detected signals from each element of the array are typically quite small, they must be amplified by a relatively large gain before being routed off-chip, and of necessity before analog-to-digital conversion. As a result, the DC offsets due to detector nonuniformities are also amplified by a relatively large gain. This requires the analog-to-digital converter to have an extremely large dynamic range in order to accommodate the large signal range caused by the amplified detector element nonuniformities. Such high resolution analog-to-digital converters, however, add cost to the system electronics.
Also, the larger signal range requires the entire digital signal processing electronics to accommodate a larger overall bit value, further increasing the system cost. Also, since the image data for each pixel must be separately compensated for offsets due to nonuniformities at the scan rate of the array, very high bandwidth electronics are required to perform the digital offset correction. This, in turn, adds further cost to the system.
Alternatively, the signal dynamic range of the detector may be artificially limited of the amount of amplification provided to the detector signal prior to analog-to-digital conversion restricted. Both these alternatives have disadvantages, however, in that the image quality and/or signal-to-noise ratio are reduced.
Accordingly, it will be appreciated that a need presently exists for a way to reduce the effects of nonuniformities in infrared focal plane arrays and to thereby increase image quality in a focal plane array imaging system. It will further be appreciated that a need presently exists for such a solution which does not add significantly to the cost of the overall infrared imaging system and which is compatible with the processing constraints of hybrid or monolithic focal plane arrays.